In recent times, nano-sized metal chalcogenides materials have been the subject of significant research due to their potential applications as biological markers, nonlinear optical materials, luminescent devices, photodetectors, catalysts, and chemical sensors, etc. One known method to prepare metal chalcogenide nanomaterials involves reaction in a confined medium such as a microemulsion or a polymer matrix whereby difficulty has been encountered in producing particles of uniform sizes.
Although a large variety of synthesis approaches have been reported for the preparation of crystalline metal chalcogenides, the large-scale synthesis or mass production of nano-sized metal chalcogenides is still a challenge. Metal chalcogenides can be prepared using a variety of wet-chemical methods including sol-gel, co-precipitation, and hydrothermal synthesis.
The extensively applied sol-gel or co-precipitation procedures are based on the hydrolysis and condensation of metal halides or metal alkoxides as precursors in aqueous solution. However, these methods suffer some major drawbacks. For example, the as-synthesized precipitates are amorphous and subsequent heat treatment is necessary to induce crystallization. This additional step results in alteration, mainly particle growth, or even in destruction of the particle morphology. Further disadvantages of aqueous systems are difficulties in controlling reaction parameters, such as the hydrolysis rate of the metal alkoxides, pH, method of mixing, rate of oxidation or sulfuration and the nature and concentration of anions. Also, a high temperature is needed (higher than 500° C.) to calcine intermediates produced during the synthesis to obtain crystalline metal chalcogenides. This has a negative effect on the particles because sintering leads to the formation of micro-sized aggregates requiring a further step of grinding the aggregates to disperse the aggregates into their primary particles. However, the grinding step can only break down the aggregates incompletely to their primary particles.
Typically, hydrothermal synthesis is conducted with a batch type apparatus. An aqueous solution is heated up to 373-673 K and then aged for several hours or days. As the solution is heated, the metal chalcogenides become more soluble in water. A hydrothermal reaction therefore occurs to produce nuclei seeds for crystallization. Hydrothermal synthesis of nano-sized fine particles is an especially difficult problem without the use of any surfactants. In addition, the requirement for the hydrothermal reactor construction material to withstand the elevated temperature and pressure of hydrothermal synthesis adds to the production cost.
One method for synthesizing nano-sized superfine particles involves using a rotating packed bed (RPB) reactor to increase micro-mixing and mass transfer. For example, it was possible to obtain hydroxide, carbonate and oxalate (Fe(OH)2, Zr(OH)4, Al(OH)3, Zn(OH)2, CaCO3, BaCO3, SrCO3, BaSn(C2O4)2. ½H2O, CaZrO(C2O4).4H2O, BaTiO(C2O4).2H2O) through reactive precipitation, followed by calcination to form nano-sized oxidated particles. However this method could not synthesize oxidated nano-sized particles directly.
Another method provides carrying out a gas-liquid reaction in a rotating packed bed reactor to produce nano-sized zinc sulfide particles. The reaction, however, uses hydrogen sulfide gas as one of the reactants, which is a potential source for environmental pollution.
There is therefore a need to provide a process that overcomes, or at least ameliorates, one or more of the disadvantages described above.